Is there no way I can get a refund?

I misread one particular item, and bought it. (It was an online code) But after realizing it, I wanted to cancel the transaction but I couldn’t.

How can I prove that I didn’t use that particular code in order to get a refund? (it was a code for an eShop game) as I really did not want to waste the money on the thing I accidentally bought >_> lol

If you saw a code, there is no way to get refund anymore.